How can a cell "know" to respond to the presence or absence of an enzyme? What is it that induces genes to work only when needed?
Using a fine baby's hair, taken from his own daughter, Hans Spemann tied off and separated the two halves of a two-celled newt embryo. The cells on either side of the knot gave rise to normal newt tadpoles. When Spemann divided the egg differently, by tying it perpendicular to the furrow between the two cells of the embryo, he obtained a dramatically different result. Only one side made a normal tadpole, while the other made a disorganized mass of belly tissue. This eventually lead to the recognition that a region of the embryo, called the dorsal lip of the blastopore, was critical for the organization of the embryo. If this region of the embryo was removed, the embryo formed a blob of tissue lacking structures that normally form on the top (dorsal) side of the animal. In 1924, Spemann proved that a graft could induce host tissues adjacent to it to completely change their fate and to form a second embryo in relation to the graft. If the dorsal lip was transplanted to the presumptive belly region of another developing embryo, it organized a second embryonic axis, and two embryos formed that were joined together. Spemann dubbed this region the "organizer" because he deduced that it organized the dorsal parts of the embryo into neural structures and could induce development of another embryonic axis. All organizers share the property of influencing the formation of pattern, or morphogenesis, in tissues or cells. The basic interpretation of their special activity is that the cells of organizers produce substances that can influence the development of other cells. Such substances have been dubbed morphogens. It has long been thought that morphogens produced in one site diffuse outward and form concentration gradients from their source. The idea then is that cells surrounding the source respond to the amount of morphogen they experience. The affected area is also called the zone of polarizing activity (ZPA). Recent advances in embryology have correlated these zones with the expression of specific genes (toolkit genes)
In the 1960's Francois Jacob and Jacques Monod, along with Andr Lwoff, shared the Nobel prize for their work on induction in the intestinal bacerium Escherichia coli . E. coli loves the simple sugar glucose, but it can break down and use other sugars if glucose is not available. Lactose is a sugar that is broken down into glucose and galacticose by an enzyme called beta-galactosidase. When E.coli is grown on glucose, very little beta-galactosidase is present. But when lactose is added to the bacterial culture, the rate of enzyme production is increased a thousand-fold almost immediately.
In another usage of the term, development which is internally governed is distinguished from that which is externally induced.